The present invention relates to a disc brake, particularly for a heavy road vehicle, having a caliper arranged in the vicinity of at least one brake disc mounted on a wheel axle of the vehicle.
Pneumatically operated disc brakes for heavy road vehicles, such as buses, trucks and trailers, have become more customary during recent years. It has appeared that the tough requirements on such brakes have led to designs that differ rather drastically from disc brake designs used for lighter vehicles and most often hydraulically operated.
The development at the present time is towards more use of electric equipment and electric control for more and more functions on the vehicles and system integration in the vehicles in order to achieve enhanced performance, for example dynamic stability control, rollover protection, and collision avoidance.
Also, by the use of electric power it is possible to dispense with air tanks and similar equipment.
It is thus quite natural that the possibility to use electric power also for braking a heavy road vehicle provided with brake discs is being discussed.
An electric disc brake may have a rotary electric motor as its means for creating mechanical power. This rotational movement has to be transformed into a linear movement for brake application. The brake application movement can be divided in a portion before a brake pad reaches the brake disc, in which the movement can occur with high speed and low force, and then the portion with the braking itself, in which only little movement with high force shall occur.
The main object of the invention is to attain means for creating such a movement in an effective, reliable and cost-effective way.
This object is according to the invention attained by a thrust rod for transforming a rotational ingoing movement from a drive meansxe2x80x94the electric motorxe2x80x94into a linear outgoing movement for transmission to the disc brake pad, intended for braking engagement with the brake disc, the thrust rod having
a non-rotatable member, preferably a spindle,
a rotatable nut member in thread engagement with the non-rotatable member,
a thrust rod gear for receiving the ingoing rotational movement, and
operationally interposed between the nut member and the thrust rod gearxe2x80x94a biased roller and ramp arrangement for rotating the nut member on the non-rotatable member at a low counterforce in the non-rotatable member and for axially transferring the nut member with the non-rotatable member at a high counterforce in the non-rotatable member.
In a practical and preferred embodiment of the inventionxe2x80x94with the non-rotatable member being a spindlexe2x80x94the thrust rod gear is rotatably arranged on the nut member, and rollers with axes radial to the spindle are arranged between radial parallel ramp surfaces, having a pitch or inclination and being arranged on the thrust rod gear and the nut member, respectively.
The roller and ramp arrangement is preferably spring biased, either by means of a compression spring or a coiled spring.
The pitch of the ramp surfaces may simply be constant over their lengths, but in order to adopt the function after the characteristics of the motor the pitch can vary over the their lengths. Especially, the pitch can be larger in the beginning of the ramp surfaces than in the remainder thereof.
It is necessary for a proper function that the rollers return to their intended original or start positions between the ramps after each brake cycle. If not, a new brake application may not obtain full capacity.
This may according to the invention be attained by means for transmitting the force of the compression spring to the nut only in the rotational direction of the latter for a return stroke of the disc brake.
Practically, this may be attained in that the compression spring is arranged between the caliper and an end plate attached to a flanged shaft, extending into the nut, an axial bearing being arranged between the nut and the flanged shaft and a one-way coupling being arranged between the nut and the flanged shaft.
With this design the nut rotates on the bearing without any influence from the one-way coupling during a brake application, whereas relative rotation between the nut and the flanged shaft will be blocked by the one-way coupling during a return stroke. Hereby, the rollers will be forced back to their original positions, before the nut starts to rotate.
As a modification, the non-rotatable member may be a sleeve, in which the nut is arranged.